An injection molding technique is a technique whereby a resin-molded article can be obtained by injecting a molten resin into a cavity of a mold and then cooling the molten resin to a solid. If the flowability of the molten resin declines at the time of injection of the molten resin, short fill of the molten resin occurs inside the cavity. Short fill of the molten resin can result in degraded quality of the resin molded article. Accordingly, prior to injection of the molten resin into the mold cavity, it is necessary to raise the temperature of the cavity surfaces in order to increase the flowability of the molten resin inside the cavity.
Heating means disclosed, for example, in Japanese Patent Application Laid-Open Publication No. H08-090623 (JP-A H08-090623 A) are known means for raising the temperature of cavity surfaces in the prior art. FIG. 12 hereof illustrates a basic arrangement of the heating means disclosed in JP-A H08-90623 A.
As shown in FIG. 12, the prior art heating means 400 comprises: an induction heating device composed of an upper-side high-frequency induction coil 401 of a conductor wire of spiral form wound clockwise from the center to the outside; a lower-side high-frequency induction coil 402 of a conductor wire of spiral form wound counterclockwise from the center to the outside and connected to the center of the upper-side high-frequency induction coil 401; and a high-frequency power supply 405 connected to the upper-side high-frequency induction coil 401 via a lead wire 403, and connected to the lower-side high-frequency induction coil 402 via a lead wire 404.
FIG. 13 hereof illustrates a method for using the aforedescribed high-frequency induction coils. A moving mold 407 is separated from a stationary mold 406, and the upper-side high-frequency induction coil 401 and the lower-side high-frequency induction coil 402 are inserted between the cavity surface 408 of the stationary mold 406 and the cavity surface 409 of the moving mold 407. Prior to injecting molten resin into the cavity of the mold, the cavity surfaces 408, 409 are heated with the heating means 400.
FIGS. 14A and 14B illustrate an operation of the above-described high-frequency induction coils. In FIG. 14A, the coils are illustrated using only the high-frequency induction coil 401. An electrical current is caused to flow through the high-frequency induction coil 401 by the high-frequency power supply 405 (FIG. 13), generating a magnetic field. Once the magnetic field is generated, lines of magnetic force are generated in the high-frequency induction coil 401 as indicated by the arrows (1).
FIG. 14B depicts a relationship of position in the diametrical direction of the high-frequency induction coil 401 and an intensity of the magnetic field. The intensity of the magnetic field reaches maximum in proximity to a second intermediate part 413 and a third intermediate part 414, and reaches minimum in a center space part 417. In other words, the intensity of the magnetic field generated by the high-frequency induction coil 401 is not constant with respect to position in the direction of coil diameter.
When the cavity surfaces 408, 409 are heated by the heating means 400 depicted in FIG. 13, at the cavity surfaces that correspond to the second intermediate part 413 and the third intermediate part 414 of the high-frequency induction coil 401, the amount of generated eddy current is greater due to high intensity of the magnetic field. On the other hand, at the cavity surfaces that correspond to an innermost part 411, an outermost part 416, and the center space part 417 of the high-frequency induction coil 401, the amount of generated eddy current is smaller due to low intensity of the magnetic field.
Where the amount of generated eddy current differs, a differential arises in Joule heat. If molten resin is injected and molded in a cavity having a heat differential among regions, flaws such as warpage, deformation, and the like, are generated in the resin molded article. Accordingly, there is a need for a molding technique by which molding quality can be improved.
In die casting or injection molding, a material is injected in a molten state into the cavity of a mold, and a molded article is obtained by cooling the material to a solid from the molten state. If the flowability of the material declines at the time that the material is injected in a molten state, the material will solidify before reaching the corners of the cavity. This gives rise to short fill of the material, leading to poorer quality of the molded article, necessitating measures to promote flowability of the material.
In order to promote flowability of the material, molding apparatuses in which measures are implemented for this purpose are known, for example, in Japanese Domestic Republication 2007-535786. FIG. 15 depicts a basic configuration of this prior art molding apparatus.
In FIG. 15, the molding apparatus 500 is composed of an upper die 501; a lower die 502; a high-frequency induction coil 505 disposed in proximity to an outer wall face 503 of the upper die 501 and an outer wall face 504 of the lower die 502 so as to encircle the outer perimeter of the upper die 501 and the lower die 502; and a high-frequency power supply connected to the high-frequency induction coil 505.
FIG. 16 depicts the action of the molding apparatus 500 depicted in FIG. 15. According to FIG. 16, in a state with the upper die 501 separated from the lower die 502, a current is caused to flow into the high-frequency induction coil 505 by the high-frequency power supply, and a magnetic field is generated in the high-frequency induction coil 505. In one moment of the time a magnetic field is generated, an eddy current is generated in the upper die 501 in the manner of the arrow (1), while an eddy current is generated in the lower die 502 in the manner of the arrow (2). Because the upper die 501 and the lower die 502 have electrical resistance, Joule heat is generated by the eddy currents and electrical resistance, and the upper die 501 and the lower die 502 are heated by this heat. If the upper die 501 and the lower die 502 are heated before material is injected in a molten state into the cavity, the flowability of the material is increased, and molding defects can be eliminated.
The high-frequency induction coil 505 is wound in proximity to the outer wall face 503 of the upper die 501 and the outer wall face 504 of the lower die 502. It is accordingly necessary to take measures so that the high-frequency induction coil 505 does not impede the movement of the upper die 501 when the upper die 501 is lifted further in order to open the die, making the system cumbersome to use. Accordingly, there is a desire for a molding technique whereby operability can be further improved.